I. Field of the Invention
The present invention relates to interferometry, in particular to an external beam expander for use with an interferometer.
II. Description of Related Art
Interferometry is the use of interference phenomena for measurement purposes, either for very small angles or for tiny distance increments (the displacement of two objects relative to one another). An interferometer is an instrument that measures distance in terms of wavelength; it can be used to determine wavelengths of particular light sources.
Virtually all interferometers operate on the same basic principle: From a beam of light coming from a single source (such as, e.g., a laser), two or more flat mirrors, partially reflective mirrors, or beam splitters are used to split off (or “pick off”) different light beams. These beams are then combined to interfere with each other. The desired result is to find alternating bands of light and dark, called fringes. Fringes are bright where the beams are constructively adding together and dark where they are canceling each other out. Based on this interference, measurements can be carried out.
A variety of interferometers are available with different optical configurations, which help to achieve the location of fringes. These configurations include for example, Fizeau, Fabrey-Perot, Michelson, and Twyman-Green. Fizeau interferometers, also known as image-plane interferometers, utilize a reference and test light path of unequal distance. Fabrey-Perot interferometers are frequency-tuning devices that exploit the properties of interference between two adjacent flat, parallel surfaces. Michelson interferometers are constructed using a half-silvered mirror inclined at a 45° angle to the incoming beam. Half the light is reflected perpendicularly and bounces off a beam-splitter; the other half passes through and is reflected by a second beam-splitter. Light passing through the mirror must also pass through an inclined compensator plate. The beam splitter and mirror arrangement of Twyman-Green interferometers resembles that of a Michelson interferometer, but a Twyman-Green interferometer is illuminated differently. While Michelson interferometers use an extended light source, the Twyman-Green interferometers use monochromatic point sources, located at the principal focus of lens.
In general, the reference surface in an interferometer separates the beam into two beams, a test beam, and a reference beam. The two beams of light travel different paths. The test beam leaves the interferometer and interacts with a surface or surfaces under test. In typical commercial applications, the surfaces under test are at succinct stages of a manufacturing process and are uncoated, resulting in a small reflection available to the interferometer for testing.
Large aperture interferometry has many commercial applications. Typically, large-scale interferometry utilizes only a small beam size, greatly limiting the use of this measurement technique. For instance, in many optical applications, such as large deformable mirror testing and large flat mirror testing, large area surface testing requires beam sizes larger than the typically commercially available beam size of 4 inches. Use of an interferometer in such optical applications requires a beam expander, which increases the size of the beam after exiting the interferometer. Existing beam expanders that convert the output of an interferometer, for example from four (4) to twelve (12) inches in diameter, consist of an off-axis mirror configuration that is cumbersome, difficult to align and maintain. Such an arrangement limits the utility of the interferometer in many test environments.
A typical Fizeau interferometer such as disclosed in U.S. Pat. No. 6,965,435 to Ina, has an internal beam expander where the final surface optical is not flat, cannot be, and is not used as a reference surface.
U.S. Pat. No. 5,561,525 to Toyonaga et al. discloses an interferometer with an internal and external beam expander. The external beam expander only expands the light leading to the reference surface, and neither beam expander lens is optically flat to act as a reference surface.
U.S. Pat. No. 4,188,122 to Massie et al. discloses an interferometer, which has two beam expanders. Neither beam expander, however, has a final surface that is flat enough to act as a reference surface.
Thus, there remains a need for a beam expander suitable for large aperture interferometry that is compact and that can be interfaced to existing phase-shifting interferometers.